This application claims the priority of Japanese application 9-005292, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a measuring element for a mass air flow meter and a mass air flow meter and, more particularly, to an apparatus suitable for the measurement of an air flow rate taken into an internal combustion engine.
In the prior art, a thermal type air flow meter which can directly detect an amount of a mass air flow is provided in electronically controlled fuel injection equipment for an internal combustion engine of an automobile, etc., for measuring the amount of intake air. Above all, attention is given to a mass air flow meter and a measuring element used therefor, manufactured by using the semiconductor micro-machining technique, from view points of the reduction of cost and the desire for a low electric power consumption. The mass air flow meter in which the conventional semiconductor substrate is used is disclosed in Japanese Patent Application Laid-Open No. 60-142268 (1985). The technique disclosed in the above publication places the accent on the reduction of manufacturing cost.
Therefore, the above-mentioned prior art fails to consider an air flow rate-temperature characteristic when the amount of intake air is measured. Namely, there is a problem that the accuracy of the measurement of the air flow rate is insufficient.
The problem of the above described conventional technique is explained with reference to FIG. 16 which is a plan view of the conventional measuring element for a mass air flow meter. In FIG. 16, reference numeral 1 designates a measuring element for the conventional mass air flow meter. The measuring element 1 has two bridges 23a, 23b formed by insulator film, and bridging air spaces 22a, 22b, 22c formed by anisotropic-etching a semiconductor substrate such as a silicon substrate. The bridge 23a is positioned at an upstream side of the direction of an air flow, and the bridge 23b at a downstream side.
A heating resistor 20 is arranged so as to sandwich the air space 22c between the two bridges 23a, 23b. Temperature sensing resistors 21a, 21b are arranged the sides of the heating resistor 20. Further, an air temperature compensating resistor 4a for measuring the air temperature is arranged at a portion of the upstream side of the air space 22a. Because the air spaces 22a, 22b, 22c are formed by anisotropic-etching the semiconductor substrate, through the opening portion of the electrically insulated film, they are connected continuously to one another below the bridges 23a, 23b. In such an air flow meter, the heating resistor 20 is heated to a predetermined temperature higher than the air temperature determined by the temperature compensating resistor 4a. The amount of air flow is measured from the difference of temperature between the upstream temperature sensing resistor 21a and the downstream temperature sensing resistor 21b, using the cooling effect of the air stream.
In the prior art constructed as described above, the variation of the temperature of the intake air is compensated only by a bridge circuit constructed by using the temperature compensating resistor 4a and the heating resistor 20. Namely, in the prior art, the temperature-dependent variation of the physical properties of air, e.g., the density, the dynamic viscosity and the thermal conductivity, is not considered. Therefore, a suitable air flow amount--air temperature characteristic can not be obtained. Further, in the event that the sensor is used in harsh environments such as in an automobile, the heat due to the increase of the temperature of the internal combustion engine is conducted to the heating resistor 20, the temperature compensating resistor 4a and the temperature sensing resistors 21a, 21b, and thus has adverse effects on the measurement accuracy. This problem also has not been solved in the prior art.
An object of the present invention is to provide to a measuring element for a mass air flow meter and a mass air flow meter which can measure the amount of air flow with a high degree of accuracy.
To attain the above-mentioned object, the present invention preferably has the following configuration.
According to a first aspect of the present invention, in a measuring element for a mass air flow meter, a temperature sensor for detecting a typical temperature indicative of the impact of heat on a heating resistor, is formed on a heat transfer pass from a configuration member supporting a substrate on which the heating resistor for measuring an air flow rate is formed. The heat transfer pass is positioned at the support portion of the substrate supported by the configuration member, or at a portion between that support portion and the heating resistor.
According to another aspect of the present invention, a measuring element for a mass air flow meter is provided with a temperature sensor for detecting a typical temperature indicative of the impact of heat on a heating resistor, formed on a heat transfer pass from a configuration member supporting a substrate on which the heating resistor for measuring an air flow rate is formed, and an air temperature sensor for detecting air temperature, formed on the substrate and out of a line of the air flow touching the heating resistor.
According to a further aspect of the present invention, a mass air flow meter has a measuring element for a mass air flow meter including a temperature sensor for detecting a typical temperature indicative of the impact of heat on a heating resistor, formed on a heat transfer pass from a configuration member supporting a substrate on which the heating resistor for measuring an air flow rate is formed; an operating circuit for supplying a current to the measuring element and detecting a signal indicative of the amount of the air flow; a control circuit for calculating the amount of the air flow based on the signal indicative of the amount of the air flow; and a memory for storing the compensation data for an air flow rate-substrate temperature characteristic. The control circuit compensates the amount of the air flow by using the compensation data, and the typical temperature obtained from the substrate-temperature sensor.
According to a further aspect of the present invention, a mass air flow meter has a measuring element for a mass air flow meter including a temperature sensor for detecting a typical temperature indicative of the impact of heat on a heating resistor, formed on a heat transfer pass from a configuration member supporting a substrate on which the heating resistor for measuring an air flow rate is formed; an operating circuit for supplying a current to the measuring element and detecting a signal indicative of the amount of the air flow; a control circuit for calculating the amount of the air flow based on the signal indicative of the amount of the air flow; and a memory for storing the compensation data for an air flow rate-substrate temperature characteristic and an air flow rate--air temperature characteristic. The control circuit compensates the amount of the air flow by using the compensation data, the air temperature obtained from the air-temperature sensor and the typical temperature obtained from the substrate-temperature sensor.
These and other features of the invention that will be apparent to those skilled in the art from the following detailed description of the invention, taken together with the accompanying drawings.